1. Field of the Invention
This invention relates to stuffing box packings, and more particularly to a three-ring stuffing box packing set for forged valves.
2. Background of the Related Art
Stuffing box packing rings for sealing moving shafts are well known in the art. In general, they are used both as a fixed bearing and to prevent leakage in a stuffing box. Stuffing boxes are commonly employed with reciprocating shafts such as piston rods and rotating shafts such as valve stems.
Packing rings are typically formed from materials that have a capacity to prevent leakage, are non-abrasive and have a low coefficient of friction. A particularly effective material of construction for stuffing box packing rings is flexible graphite tape as it satisfies each of these criteria. Flexible graphite tape has one negative attribute however, in that it tends to extrude through fine clearances as a result of heavy compressive forces that are applied during packing installation. In the past, to control and prevent extrusion, packing assemblies have included anti-extrusion end rings which are often formed from braided packing materials, or the like.
An example of an extremely effective prior art stuffing box packing system for sealing the stuffing box of a valve stem is disclosed in U.S. Pat. No. 4,328,974 to White et al. the disclosure of which is herein incorporated by reference in its entirety. The packing system comprises first and second sealing assemblies separated from one another and supported between flat rigid spacer rings. Each sealing assembly includes a low density graphite preform ring and a higher density die-forned graphite adapter ring. Each preform ring has a density of about 1.1 g/cc to about 0.7 g/cc and each adapter ring has a density of about 1.4 g/cc to about 1.7 g/cc. The facing surfaces of the rings are each oriented at an acute angle to the longitudinal axis of the packing, with the angle in one surface being about 45.degree. and the other being about 60.degree..
During packing installation, the differing densities and the difference in angles between the facing surfaces of the high density adapter rings and the low density preform rings provide a predictable stress, that causes the material to flow and expand in a controlled manner through deformation, forming an effective seal around the valve stem. The packing assembly disclosed in the White et al. patent also includes anti-extrusion braided packing rings which act as wipers to strip graphite particles from the valve stem and absorb them into the braid.
A more recent example of a stuffing box packing assembly for valve stems, shafts and piston rods is disclosed in U.S. Pat. No. 5,806,858 to Harrelson, III, the disclosure of which is also herein incorporated by reference in its entirety. The packing is a five-ring assembly consisting of three nested graphite rings of equal size rigidly supported between two braided anti-extrusion end rings. The graphite rings are die-formned from flexible graphite tape and have a density of about 0.5 g/cc to about 1.4 g/cc, and the braided end rings are die-formed from reinforced braided-stock and have a density in excess of 1.8 g/cc. The lower density graphite rings have facing surfaces oriented at a 45.degree. angle, and the higher density end rings have facing surfaces oriented at 60.degree. angles, so as to cause the lower density material to flow and expand during installation. Advantageously, the braided end rings, being relatively rigid, are unaffected by the compressive forces applied during packing. They serve as wiper rings to remove particles of graphite from the shaft or valve stem and provide extrusion protection for the packing.
Industrial piping systems generally employ several different types of valves for performing various fluid control tasks. Some industrial valves are relatively expensive and are often reconditioned after a certain period of service. Other valves, such as forged valves, are less expensive, and are generally discarded after a certain period of service due to wear. In such instances, the use of relatively expensive valve packing sets, such as the eleven and seven-ring packing sets disclosed in U.S. Pat. No. 4,328,974 to White et al. or the five-ring packing set disclosed in U.S. Pat. No. 5,806,858 to Harrelson, III, are not economical. Indeed, in such instances such complex valve packing sets may cost as much or more than the valve itself.
Therefore, it would be beneficial to provide an inexpensive stuffing box packing set for use in conjunction with a forged valve stem, or the like. Such a packing set would not require the use of anti-extrusion rings, wiper rings, resiliency rings, spacer rings or the like.